During anaerobic digestion, ammonium is present in waste streams such as slaughterhouse wastewater, protein-rich biomass and other biomass streams can become toxic due to an increased ammonium concentration in combination with elevated pH levels. This ammonium needs removal from the digester in order to achieve successful biogas production. In the past several approaches have been presented, principally including the addition of alkaline solutions, such as caustic, and subsequent acids to strip out the ammonia and capture it. These approaches entail high chemical costs as well as addition of considerable quantities of ions to the digester fluid. There is hence a need for a technology able to separate ammonia without consumption of expensive chemicals for stripping and/or capture.
Several studies further described the removal of ammonium from waste streams via transport towards the cathode and subsequent volatilization following high pH. However, in these studies the ammonium flux was not selectively enhanced by the cathodic accumulation of non-target ions leading to a mobility determined by electric field only besides a small gradient caused by ammonia volatilization.
Also anions, the issue with ammonium is a cation based issue, can be problematic for biological conversions. The fermentation of biomass to, e.g., bioethanol leads to the production of acetate. As the fermentation causes decreasing pH, the acetate is converted to acetic acid, which is toxic for most microorganisms. Present approaches to maintain fermentations include the addition of caustic solutions, which elevate the pH and enable further biological activity. This caustic addition is costly and leads to an increased salinity of the fermenter broth itself, which is undesired particularly in closed loop systems. There is hence a need for a technology that can separate out acetate in combination with elevating pH without considerable increase of the broth ionic strength.
Not only acetate as an anion can cause toxicity, most anionic organics formed during fermentation, such as butyric, caproic or caprylic acid can have the same negative impact. Interestingly, these chemicals have a considerable market value, if presented in a purified form. Thus, while microbial production processes exist for any of these chemicals, and in extension others such as succinate, citric acid, maleic acid, formic acid and others, these endproducts often exert some level of toxicity to the producing organism. Moreover, if present in a broth particularly coming from mixed culture fermentations or as a spent broth after a bioproduction process, the recovery of these products is as yet not economical.
A technique that allows separation of ionic products from fermentation broths is electrodialysis (ED). In ED, two electrodes are separated via a sequence of multiple, alternating anion and cation exchange membranes. This allows the removal from one solution of both anions and cations, to be captured in a concentrate solution. While apparently attractive in the aforementioned context, ED suffers from low ionic efficiency (as predominantly other ions such as sodium or chloride are separated) and are not always compatible with the viscous nature of the fermentation broths or the digester fluids. As an example on the use of electrodialysis, we refer to Boniardi and coworkers (Journal of Applied Electrochemistry 27, p 125-133 1977).
In so-called bipolar electrodialysis (Wong et al. Biotechnology Letters 32 pp. 1053-1057 2010), it has, moreover, been shown that application of electrical current to an Escherichia coli culture fermenting glucose did not negatively impact this organism, while increasing solution pH as well as transporting acetate from the broth to a counter compartment. In this compartment a recovery solution was brought. However, the process still led to a weak solution of the formed acetate in the recovery compartment. Moreover, the latter study is very similar to earlier work by Nomura and coworkers (Applied and Environmental Microbiology 54(1) pp. 137-142 1988) in which acetate production was improved from ethanol in an ED stack.
The key drawbacks from the above methods are that the products are not yet recovered at high strength, that the efficiency of the transfer from the products towards the recovery compartment is limited or that the system is not amenable to process viscous fluids as present in many bioproduction settings.